Sealants are often used in automotive, marine, aerospace, and construction markets to fill gaps, to prevent corrosion, and to create aesthetically pleasing surfaces. Commercially viable sealants strike an acceptable balance among end use performance properties such as cure speed, shelf life, rheological characteristics, adhesion to various substrates, and thermal and ultraviolet light stability. Consequently, such sealants typically contain rheology modifiers, adhesion promoters, oxidative stabilizers, plasticizers, and cure catalysts. A variety of technologies have been found useful in the manufacture of these sealants, including acrylic latexes, moisture curing silicones, and moisture curing isocyanate functional polymers. Moisture curing isocyanate functional polymers generally cure rapidly and, after cure, provide good adhesion to paint overcoats. Therefore, in applications which require that the sealant be painted soon after it is applied to a substrate, sealants based on moisture curing isocyanate functional polymer technology are particularly preferred. Even so, several problems attend the use of sealants based on moisture curing isocyanate functional polymers; under very hot, humid conditions these sealants are prone to foaming and the alcoholic co-solvents which are present in many paints react with the isocyanate termini of the moisture curing isocyanate functional polymers thereby rendering them non-reactive and permanently preventing cure.
Recently, sealants based on moisture curing alkoxysilane functional polymers have been proposed as alternative to those based on moisture curing isocyanate functional polymers. The alkoxysilane functional polymers are preferred over isocyanate functional polymers especially because they do not foam under hot, humid conditions and they cure properly even in the presence of alcoholic paint co-solvents. It is possible, therefore, to paint sealants based on alkoxysilane functional polymers before they are fully cured (i.e., wet-on-wet) thereby eliminating the non-productive cure time required before painting sealants based on moisture curing isocyanate functional polymers.
Alkoxysilane functional polymers known in the art have been prepared by numerous methods including: condensation of isocyanate functional polymers with amine, mercaptan, or hydroxyl functional silanes; condensation of amine, mercaptan, or hydroxyl functional polymers with isocyanate functional silanes; coupling of mercaptan or hydrosilane functional polymers with alkene functional silanes; and coupling of alkene functional polymers with mercaptan functional silanes or alkoxyhydrosilanes. Alkoxysilane functional polymers may be compounded to form materials useful as moisture curable sealants. While a large number of alkoxysilane functional polymers has been disclosed in the art, none provide moisture curing sealants exhibiting acceptable paint adhesion when the paint is applied during the latter stages of cure.
U.S. Pat. No. 3,627,722 (Seiter), U.S. Pat. No. 4,067,844 (Barron et al.), U.S. Pat. No. 4,857,623 (Emmerling et al.), and U.S. Pat. No. 5,364,955 (Zwiener et al.) each disclose alkoxysilane functional poly(ether urethanes). None of these teaches a moisture curable sealant exhibiting acceptable paint adhesion when the paint is applied during the latter stages of cure.
U.S. Pat. Nos. 5,476,889 and 5,464,888 (Owen) describe alkoxysilane functional polymer based sealant compositions in which the alkoxysilane endgroups are attached to a polypropylene oxide backbone through alkylene linking groups. While the initial paint adhesion of these sealant compositions was improved through incorporation of polar plasticizers such as N-ethyl-p-toluenesulfonamide or polar solvents such as N-methylpyrrolidinone, the paint adhesion decreases to unacceptable levels as the alkoxysilane functional polymer cures.